The bottom ram in a hydraulic steel mine prop includes an outer cylindrical tube with a cylindrical floor and an end collar. The top ram in such props is guided on the one hand coaxially slideably in the bottom ram by the end collar with at least one guide ring. The top ram basically includes a cylindrical tube enclosing a piston sealed off and guided relative to the inner wall of an outer cylindrical tube, a prop head, a setting and withdrawal valve, and an inner stroke limitation and a return spring.
A mine prop of the above described type has, for example, become known from the German Patent Publication (DE-OS) 3,541,871. Such mine props have been completely successful regarding their functionality. Such steel mine props are known in numerous structural variations. Due to their great weight, attempts have already been made to produce such mine props of light metal. However, such light metal mine props are not usable wherever the danger of firedamp exists, because even slight impacts on the light metal tubes can produce sparks which then can cause a gas explosion. Moreover, such light metal mine props are also about twice as expensive as corresponding steel mine props. In steel mine props of the known type, an outer cylindrical tube is welded to a cylindrical floor or a prop foot. Because this outer cylindrical tube serves as a running bushing for the inner piston of the inner cylindrical tube, normally it is machined precisely by drilling and subsequent grinding or honing. It is furthermore surface treated, for example, with cadmium or zinc. A groove is turned-in in the head region of the outer prop tube for holding the end collar with the handle , which is arranged in that region, whereby the holding is achieved by means of a groove wire, which is laid into the corresponding groove. The end collar additionally function as an extension stroke stop for the inner cylindrical tube. In order to determine the extension stroke length, a stop bushing is inserted between the inner cylindrical tube and the outer cylindrical tube, and is taken along by the piston of the inner cylindrical tube during the extension stroke movement. Depending on the length of this stop bushing, the stop bushing will abut against the stop surface of the end collar with the inner cylindrical tube driven out more or less far, whereby a further extension of the inner cylindrical tube becomes impossible. The end collar in turn represents a second guide for the inner cylindrical tube. However, the end collar must now take up the full axial force created by the prop and transfer this force through the groove wire onto the outer cylindrical tube. Due to the fact that the outer cylindrical tube is welded to a cylindrical bottom, easily weldable material must be used for this tube and therefore high strength steel alloys cannot be used. Due to the arising load, pressures of about 400 bar arise in the outer cylindrical tube, this cylindrical tube has a correspondingly large wall thickness and therewith a corresponding weight. Here it must be remembered that transportation and erection of such mine props in practice is done exclusively manually.
The valve housing for the setting and withdrawal valve is welded on at a suitable location. This welding work and the welding work by means of which the outer prop tube is welded to the bottom, produces deformations, which, on the one hand, negatively influence the tight sealing of the valve insert and, on the other hand, damage the mentioned surface treatment of the tubes.
Moreover, it is to be considered that turning-in the groove for the groove wire of the end collar weakens the outer prop tube so that a corresponding increase in wall thickness is necessary.
The internal stop sleeve can easily become encrusted and thereby prevent pushing the prop tubes together. Moreover, because this sleeve moves along with the extension and withdrawal motion of the inner prop, damage to the surface protection of the inner surface of the outer prop is unavoidable.
From the described structural features it can be seen that the known construction of steel mine props is very heavy, and it can easily be recognized that repairing such props is not possible without qualified personnel and without technical equipment including high quality machine tools. In any case, independently thereof, when making repairs at least the surface protection of the prop tubes will be damaged. Moreover, because different lengths of steel mine props are required due to the different structural heights, a suitable number of the required different lengths must always be kept in stock. It is not possible to produce such mine props according to requirements in an above-ground workshop on site. According to the above mentioned DE-OS 3,541,871, it is already seen as a great advance that a prop assortment of "only" eight different lengths must be produced for the different structural heights arising in practice.